/**
 * A very simple map extension that allows to store multiple values for the same key
 * by storing them in an array.
 *
 * @todo Add more methods if needed.
 */
export declare class MultiValueMap<K, V> extends Map<K, V[]> {
    /**
     * Adds a value to the map. If the key already exists, the value will be appended to the existing array,
     * otherwise a new array will be created for the key.
     *
     * @param key Key to add.
     * @param values Value(s) to add.
     */
    add(key: K, ...values: V[]): void;
}